1. Field of the Invention
The invention relates to a process for modifying the surface of polymer substrates by controlled graft polymerization, initiated by means of electromagnetic radiation or thermally, of ethylenically unsaturated compounds. The invention further relates to a method of using the modified polymer substrates for the production of products and to the products themselves.
2. Description of the Related Art
The modification of the surfaces of plastics, specifically of products used in industry, is of great commercial interest. The graft polymerization of ethylenically unsaturated monomers, in particular, has proven industrially and commercially significant, since by this means it is possible to find new applications for standard plastics already established in the market. Through the changes in the surfaces of the plastics it is possible to produce, in an efficient and cost-effective manner, products with interface properties optimized for the specific application. These changed properties can give, inter alla, hydrophilicized, dirt-repellent, printable and flame-retardant surfaces, and surfaces with increased solvent resistance. An overview of the varied possibilities for changing the properties of synthetic polymers by photoinitiated grafting is given by Arthur, Jr. J. C. in Dev. Polymer Photochem. 2 (1982) 39.
Various processes are known for modifying the surfaces of polymers by graft polymerizations. The grafting reaction is generally preceded by an activation of the relevant surface, i.e., either before the actual grafting or simultaneously with it, reactive centers are created on the surface of the substrate which serve as starting points for the actual polymerization as the reaction proceeds. This activation of the surface can be carried out, for example, by gamma radiation, ultraviolet radiation with wavelengths below 180 nm, plasma treatment, ozone treatment, electrical discharges, flame treatment, macroinitiators or photoinitiators.
U.S. Pat. No. 4,189,364 discloses that polymer surfaces can be modified by immersion in a solution of 2-hydroxyethyl methacrylate and dimethacrylate and irradiation with a .sup.60 Co source to create a new surface with significantly better water absorption. A disadvantage of this method is that it requires the availability of a .sup.60 Co source, with its associated complexity and cost. Furthermore, the type of radiation emitted from this source is non-specific and its effect is not restricted to the surface of the substrate to be modified, so that undesirable changes in the mechanical and chemical properties of the bulk of the polymer are caused.
The activation of a surface using ultraviolet radiation of a wavelength below 180 nm requires that, during the activation phase, oxygen is largely excluded, since it has a very strong absorption at the abovementioned wavelength. Since, on the other hand, activation by this method, which is ultimately based on the formation of oxidized reactive sites, requires at least a certain partial pressure of oxygen, it is very difficult to achieve a reproducible activation step in the context of an industrial process. In this connection, the continuous decrease in intensity of the relevant irradiation tubes also creates great problems. Besides this, an undesired change in the bulk properties of the substrate, caused by the irradiation, cannot be avoided, since highenergy radiation of this type can also break carbon-carbon bonds.
A plasma pretreatment, as described in EP 0 574 352, is likewise a method which proceeds under reduced pressure and which in practice reduces the process to a batch process, i.e., makes it extremely difficult to conduct the process continuously. Furthermore, this requires correspondingly complicated equipment, and in addition the activation is difficult to reproduce, because of the wide variety of independent plasma parameters.
The ozonization of a polymer surface to form oxidized reaction centers, as described, for example, in U.S. Pat. No. 4,311,573, U.S. Pat. No. 4,589,964 or EP 0 378 511, can only be carried out using particular protective measures, because of the toxicologically hazardous and fugitive character of ozone. For quality assurance in an industrial process, complicated control mechanisms are required for the reproducible setting of the relevant ozone concentrations, in order to ensure the consistent quality of the product produced.
Electrical discharges, as employed, for example, in the context of a corona treatment for surface activation, are generally, because of the specific requirements of the method, applicable only to substrates having a simple shape and a large surface, for example to film webs or extruded profiles. Similar considerations apply also to the flame treatment of polymers, and in this case the more severe thermal stress in particular at exposed locations of the substrate is an additional factor. A comparison of both methods, with possible and actual applications, is given, for example, by Gerstenberg, K. W. in Coating 9 (1994) 324 and Coating 10 (1994) 355.
Other ways of creating activated surfaces are provided by the application of initiator molecules, such as macroinitiators or photoinitiators.
The effect of macroinitiators is based on the application of preformed polymers having reactive groups onto the substrate to be modified. The bonding to the substrate in this case is purely physical. The actual grafting is initiated by a thermal or photochemical excitation of the relevant reactive groups of the macroinitiator. On the one hand, this method requires the synthesis of macroinitiators, which are frequently not commercially available, and on the other hand it is not always possible to ensure the permanent physical bonding of the macroinitiator to the respective substrate, even under the influence of solvents and temperature variation.
The use of photoinitiators for grafting is essentially based on a chain transfer and is universally applicable. Here, initiator radicals or polymer radicals abstract, for example, hydrogen atoms or chlorine atoms from the respective substrate and form macro radicals which initiate the graft polymerization of the added monomers. As described by H. G. Elias in Makromolekule Vol. 1 (1990) 572 ff. the achievable graft yield here is, however, very low, because of the low transfer constants of polymer radicals.
The grafting of HDPE, LDPE and polystyrene with acrylic acid and benzophenone as photoinitiator in the gas phase, described by K. Allmer et al., in J. Polym. Sc., Part A, 26, 2099-2111 (1988) is such a process with low transfer constants. It is, furthermore, unsuitable for monomers, such as sodium styrenesulfonate, which cannot be transferred into the gas phase. The method of S. Tazuke et al., described in ACS Symp. Ser. 121, 217-241 (1980), in which the polymer substrate is dipped in a solution containing the photoinitiator and the monomer and irradiated, is one of the processes in which no pretreatment of the substrate to promote grafting takes place and which therefore shows low transfer constants.
In contrast, according to H. Kubota et al., (I. J. Polym. Sc.: Polym. Ed. Lett., 19, 457-462 (1981)), PP and LDPE films are pretreated with a solution containing the photoinitiator, specifically benzophenone or anthraquinone or benzoyl peroxide, and polyvinyl acetate as carrier for the photoinitiator. By this means, the photoinitiator was physically linked to the substrate surface after removal of the solvent, namely acetone or chloroform. Methyl methacrylate, in the gas phase, and acrylic acid and methacrylic acid, in the liquid phase, were grafted onto the pretreated substrate surfaces with high yields. H. Kubota et al. in II. J. Polym. Sc.: Polym. Ed. Lett., 20, 17-21 (1982), investigated the influence of different solvents on the gas phase grafting of the monomers onto the substrate surfaces pretreated as described. A disadvantage of this process is the additional use of a film-forming agent, specifically polyvinyl acetate, as carrier for the photoinitiator. On the one hand, it is not possible without difficulty to distribute the photoinitiator with the desired homogeneity in the film-forming agent, and on the other hand it is unavoidable that the film-forming agent is also grafted onto the substrate, as a result of which the uniformity of the coating is impaired. Finally, the monomer is not only grafted onto the substrate to be modified, but also, unavoidably, onto the film-forming agent, and in the extreme case, depending on the graftability of the respective substrate, virtually exclusively onto the film-forming agent.
Z. P. Yao and B. Ranby have described a continuous process in which acrylamide or acrylic acid is grafted onto HDPE films (J. Appl. Pol Sci., 40 1647 (1990)), and for which the film is passed through a solution of monomer and benzophenone as photoinitiator in acetone as solvent ("presoaking") and irradiated. In the case of acrylamide, sublimating acrylamide vapor was also involved in the radiation-initiated grafting. The process is suitable for the coating of flat products, such as films. Preparatory modification of an irregularly shaped substrate surface is not possible. A further disadvantage is that the time for the "presoaking" and the irradiation time are rigidly linked, since the process operates continuously. A further disadvantage is that no temperature control of the "presoaking" solution is provided. The process is inflexible and lacks a number of important degrees of freedom. An optimal balance of the parameters which are decisive for success, specifically the concentration of initiator, monomer and solvent, the temperature of the solution and the duration of the "presoaking" and of the irradiation, is not possible.